Apple (AAPL) shares rose 0.7% on Monday to $129.66 after activist investor Carl Icahn called current valuation "undervalued." He said Apple is worth $240 a share. In a letter to Apple CEO Tim Cook, Icahn also called for accelerated buybacks.

"After reflecting upon Apple's tremendous success, we now believe Apple shares are worth $240 today," wrote Icahn. "Apple is poised to enter and in our view dominate two new categories (the television next year and the automobile by 2020) with a combined addressable market of $2.2 trillion, a view investors don't appear to factor into their valuation at all."

BHP Billiton (BHP) plunged 5.4% after the company's South32 spinoff was valued at around $9.1 billion, the lower end of expectations. The split will allow BHP to focus on its businesses in iron ore, petroleum and coal.

A measure of homebuilder confidence declined 2 points to 54 in May, though remained above the 50 level indicating optimism, according to the National Association of Home Builders/Wells Fargo data. The gauge was pulled lower by signs of weakness in sales of single-family homes. Economists had expected the measure to increase to 58.